1. Field of the Invention
The present invention relates to a sintered magnet which contains FeCo crystals having a high saturation flux density and contains a heavy rare-earth element unevenly distributed.
2. Description of Related Art
Japanese Unexamined Patent Application Publication (JP-A) No. 2010-74062 discloses a nanocomposite magnet including an iron-cobalt (FeCo) soft magnetic phase and neodymium-iron-boron (NdFeB) being composited with each other, but this literature does not refer to a sintered magnet. JP-A No. 2008-60183 discloses a FeCo ferromagnetic powder coated with a fluoride, but this literature does not refer to the cobalt composition of FeCo crystals. JP-A No. 2006-128535 describes the atomic ratio between cobalt and Fe in a magnetic powder containing Fe and Co, but does not refer to an NdFeB sintered magnet. JP-A No. 2001-68319 discloses a ferrite magnet having a microstructure containing cobalt unevenly distributed, but does not refer to an NdFeB sintered magnet. JP-ANo. 2001-274016 discloses a rare-earth alloy film magnet having a periodically varying concentration of a specific element in the magnet, but does not refer to FeCo crystals. JP-ANo. 2007-294917 discloses a sintered magnet containing a heavy rare-earth element being unevenly distributed and enriched at peripheries of crystal grains, but lacks description about FeCo crystals.
These customary techniques, however, do not give magnets having a maximum energy product higher than the theoretical maximum energy product (64 MGOe) of Nd2Fe14B and fail to provide a high-density magnet which allows both improvement in the maximum energy product and reduction in amount of a rare-earth element. The technique of unevenly distributing a heavy rare-earth element in an NdFeB magnet, when employed alone, does not contribute to the reduction in amount of a rare-earth element. The technique of mixing with a soft magnetic powder and sintering the resulting mixture results in a magnet having an insufficient coercivity and remarkably inferior heat resistance or resistance to demagnetization.